Fluids play a pivotal role in many earth science problems. The most common tool used to analyze the fluid content in rocks and sediments is measurement of seismic (compressional and shear) wave velocities in the earth. Liquids reside underground by saturating--or only partially saturating--existing void space (porosity) in rock or soil. Two of the most important, and most difficult to measure, properties of subsurface rocks and fluids are porosity and saturation level.
Resolution of various practical and scientific issues in the earth sciences depends on knowledge of fluid properties underground. In environmental cleanup applications, the contaminant to be removed from the earth is often a liquid such as gasoline or oil, or ground water contaminated with traces of harmful chemicals. In commercial oil and gas exploration, the fluids of interest are hydrocarbons in liquid or gaseous form. In analysis of the earth structure, partially melted rock is one key to determining temperature and local changes of structure in the mantle. In all of these cases the most common tool used to analyze the fluid content is measurements of seismic (compressional and shear) wave velocities in the earth. The sources of these waves may be naturally occurring such as earthquakes, or man-made such as reflection seismic surveys at the surface of the earth, vertical seismic profiling, or still more direct transmission measurements using seismic/ultrasonic logging tools in either shallow or deep boreholes.
Underground fluids occupy voids between and among the solid earth grains. When liquid or gas completely fills interconnecting voids, a well-known result due to F. Gassmann's analysis "Uber die elastizitat poroser medien." Vierteljahrsschrift der Naturforschenden Gesellschaft in Zurich, 96, 1-23 (1951) predicts how the composite elastic constants that determine velocities should depend on the fluid and drained rock or soil elastic constants and densities. Gassmann's result is a low frequency calculation and both laboratory and well-log measurements of wave velocities at sonic and ultrasonic frequencies have been observed to deviate markedly from Gassmann's predictions. This is especially so for partial saturation conditions (i.e., when the fluid in each pore is a mixture of gas and liquid). In some cases these deviations can be attributed to "patchy saturation," meaning that some pores are fully saturated with liquid and others are filled with gas, so that Gassmann's formulas apply locally (but not globally) and must be averaged over space to obtain the overall seismic velocity of the system. In other cases, neither Gassmann's formulas nor the "patchy saturation" model seem to apply to seismic or ultrasonic data. In these cases a variety of possible reasons for the observed velocity discrepancies have been put forward, including viscoelastic effects (velocity decrement due to frequency-dependent attenuation), fluid-enhanced softening of intragranular cementing materials, chemical changes in wet clays that alter mechanical properties, etc.
Previous attempts at using seismic velocity data to determine the state of saturation of the earth have only been partially successful. A method employed for about the past 15 years has used changes in wave amplitude from seismic reflection surveys to infer the presence of fluid changes at interfaces. Such methods have been termed "amplitude versus offset" or AVO which is an advanced version of "bright spot analysis" methods employed in the late 1970's. Such methods have been limited to reflections and require interfaces between high contrast materials. Furthermore, previous methods have not used robust measures, i.e., measures that cannot be easily corrupted by either simple data errors or over-dependence on theoretical assumptions.
Presently, a need still exists to find a method of using seismic data obtained from either a transmission mode or reflection mode, but not necessarily requiring interfaces or reflections to estimate porosity and saturation. The method should be applicable regardless of whether the rock or soil fits the Gassmann, the patchy saturation, or some other model that is less dependent on the underlying causes of the above discrepancies, and thereby provides a robust means of discriminating the state of partial saturation, whatever the other effects might be that are caused by the presence of the fluid.